Elastic surface wave functional device and electronic circuit using the element

ABSTRACT

A surface acoustic wave functional element is provided that includes a piezoelectric substrate or a multilayer piezoelectric substrate having a large electromechanical coupling coefficient. Semiconductor layers are formed on the piezoelectric substrate. The semiconductor layers include an active layer and a buffer layer. The buffer layer is formed of a structure that has a lattice constant that is the same as or similar to that of the active layer. In addition, input and output electrodes are formed on both sides of the semiconductor layers. The surface acoustic wave functional clement attains a large amplification gain at low voltage, and can be used as part of a transmitting/receiving circuit in a high frequency portion of a mobile communication device.

TECHNICAL FIELD

The present invention relates to surface acoustic wave functionalelements, such as a surface acoustic wave amplifier and a surfaceacoustic wave convolver, in which surface acoustic waves propagating ina piezoelectric substrate interact with carriers in a semiconductor, andto an electronic circuit including such a surface acoustic wavefunctional element.

BACKGROUND ART

Recently, a mobile communication apparatus such as a portable telephonehas been down-sized and devised to operate at lower voltages withreduced power consumption. With this progress, intensive investigationhas been made to develop monolithic elements which can be mounted in aportable apparatus. However, since a bandpass filter and a duplexer arebigger in size than other high frequency components, there is littleadvantage to fabricate these elements monolithically together with otherelements. Moreover, it is very difficult to fabricate a power amplifieras a monolithic element. For this reason, a duplexer, a power amplifier,a bandpass filter, a low noise amplifier arranged upstream of thebandpass filter, etc. have been developed as respective discreteelements and fabricated as respective modules. When these discreteelements are fabricated as modules, wiring for connecting a plurality ofparts and circuitry for matching impedance are formed, and therefore thediscrete elements as units are very large in size.

On the other hand, there have been made various studies on amplificationof surface acoustic waves. In order to amplify the surface acousticwave, it is known to propagate the surface acoustic wave in a surface ofa piezoelectric substrate and couple the electric field generated by thewave with carriers in a semiconductor. Actual surface acoustic waveamplifiers are classified into three types according to the types ofcombination of the piezoelectric material for propagating the surfaceacoustic wave and the semiconductor: (1) a direct type amplifier (FIG.3); (2) a separation type amplifier (FIG. 4); and (3) a monolithic typeamplifier (FIG. 5). As shown in FIG. 3, the direct type amplifier is anamplifier having the structure which has a substrate 7 composed of amaterial, such as CdS or GaAs, with both piezoelectric characteristicsand semiconductor characteristics simultaneously, on which input andoutput electrodes 4 and 5 are provided, with the substrate 7 beingsandwiched by electrodes 6 for applying a direct current electric fieldto the substrate 7. However, a piezoelectric semiconductor with largepiezoelectric properties and large mobility has not been found so far.As shown in FIG. 4, the separation type amplifier is an amplifier havingthe structure in which a semiconductor 3' of a large mobility isdisposed on a piezoelectric substrate 1 of large piezoelectric propertywith a gap 8. Input and output electrodes 4 and 5 are provided on thesubstrate 1, and electrodes 6 for applying a direct current electricfield to the semiconductor 3' are provided on both sides of thesemiconductor 3'. In the amplifier of this type, surface flatness of thesemiconductors and the piezoelectric substrate and the size of the gap 8have a great effect on the amplification gain. In order to obtain apractically acceptable amplification gain, the gap 8 must be made assmall as possible and maintained constant over an operation range and sothat industrial fabrication of the amplifiers with such a gap is verydifficult. As shown in FIG. 5, the monolithic type amplifier is anamplifier having the structure in which a semiconductor 3' is formed ona piezoelectric substrate 1 via a dielectric film 9 without a gap. Inputand output electrodes 4 and 5 are provided on the piezoelectricsubstrate 1, and electrodes 6 for applying a direct current electricfield to the semiconductor layer 3' are provided on both sides of thesemiconductor layer 3'. The monolithic type amplifier can achieve a highgain and be used in a high frequency region. Moreover, the monolithictype amplifier is said to be suitable for mass production. However,application of these surface acoustic wave amplifiers to a mobilecommunication apparatus such as a portable telephone has not beenstudied yet.

In order to realize a monolithic type amplifier, a semiconductor film ofgood electric characteristics must be formed on a piezoelectricsubstrate and the semiconductor film must be sufficiently thin so thatthere can occur efficient interaction between the surface acoustic waveand the carriers in the semiconductor. According to the study byYamanouchi et al. of Tohoku University in 1970s (Yamanouchi K., et, al.,Proceedings of the IEEE, 75, p726 (1975)), an electron mobility of InSbof 1,600 cm² /Vsec was achieved using the structure in which SiO iscoated on a LiNbO3 substrate to a thickness of 30 nm and then InSb thinfilm is evaporation-deposited on the substrate to a thickness of 50 nm.When a DC voltage of 1,100 V was applied to a surface acoustic waveamplifier having the semiconductor films, an amplification gain of netgain 40 dB was obtained at a center frequency of 195 MHz. Furthermore,based on their theoretical calculation, Yamanouchi et al. predicted thatin an InSb thin film of 50 nm thick, the maximum electron mobility is3,000 cm² /Vsec because of surface scattering of carriers (Yamanouchi etal., Shingaku Gihou, US78-17. CPM78-26, pl9 (1978)). That is, themonolithic type amplifier faces the trouble that a thin filmsemiconductor layer having good electric characteristics is difficult tobe formed on a piezoelectric substrate. Moreover, a conventionalstructure requires a dielectric film such as SiO in order to preventdeterioration of InSb and a LiNbO₃ substrate because of diffusion ofoxygen from the LiNbO₃ substrate. Moreover, when a surface acoustic waveamplifier is used as an amplifier of a high frequency portion of aportable apparatus and a bandpass filter, the surface acoustic waveamplifier is useless if it gives no amplifying effect at a drivingvoltage of 3 to 6 V. A conventional monolithic amplifier needs a highvoltage and there was no surface acoustic wave amplifier that could bedriven at low voltages. Furthermore, there is the problem that a surfaceacoustic wave convolver, which makes use of interaction between asurface acoustic wave and electrons like the surface acoustic waveamplifier, gives an insufficient gain.

In general, an amplification gain, G, of a surface acoustic waveamplifier is given by the following equation: ##EQU1## where A=aconstant, k² =an electromechanical coupling coefficient, εp=anequivalent dielectric constant of a piezoelectric substrate, σ=aconductivity, h=a film thickness of an active layer, μ=an electronmobility, E=an applied electric field, and v=a velocity of a surfaceacoustic wave. In order to obtain a large amplification gain at a lowvoltage of a practical level, it is necessary that; (1) a semiconductorthin film is formed which has a high electron mobility and whose filmthickness is as thin as possible; and that (2) a piezoelectric substrateis used whose k² is as large as possible.

The present inventors have made intensive investigation on the aboveproblems and as a result found that an active layer which is a thin filmand has good electric characteristics can be obtained by inserting abuffer layer between the piezoelectric substrate and the active layer.The present inventors also found that an electromechanical couplingcoefficient k² of the piezoelectric substrate far larger than that of abulk can be achieved by using a multilayer piezoelectric thin filmsubstrate of at least three layers. Furthermore, the present inventorsconfirmed that a surface acoustic wave amplifier is fabricated using thesemiconductor layer or the piezoelectric thin film substrate and thatgood amplification gains can be obtained at practical low voltages bythis amplifier, thus accomplishing the present invention. In addition,an electron mobility of 5,000 cm² /Vsec or more of the active layer hasbeen achieved with the semiconductor film structure of the presentinvention. That is, the present invention provides a surface acousticwave functional element comprising a piezoelectric substrate, input andoutput electrodes provided on the piezoelectric substrate, semiconductorlayers provided between the input and output electrodes. Thesemiconductor layers include an active layer and a buffer layer latticematched to the active layer. Here, by the term "active layer" is meant alayer which oscillates a surface acoustic wave which is being propagatedwith energy of carriers in the semiconductor.

In the present invention, the thin film active layer can have very goodelectric characteristics of an active layer because the crystallinity ofthe active layer can be improved by inserting a buffer layer between thepiezoelectric substrate and the active layer. Moreover, the presentinventors found that when the lattice constant of the crystal formingthe active layer is made equal or similar to that of crystal forming thebuffer layer, the crystallinity of the active layer can be furtherimproved, and that the electric characteristics of the active layer canbe markedly improved even when the active layer is in the form of a thinfilm. The present inventors also found that still better electriccharacteristics can be obtained by using a compound semiconductorcontaining Sb as the buffer layer of the present invention. The bufferlayer of the present invention is characterized by high resistance andsmall attenuation of a surface acoustic wave therein. The buffer layerof the present invention has superior properties in that it prevents theactive layer from being deteriorated by oxygen from the piezoelectricsubstrate even when no dielectric film such as Sio is provided on thepiezoelectric substrate and that it grows at low temperatures so that itdoes not deteriorate the piezoelectric substrate.

The piezoelectric substrate of the present invention comprises amultilayer piezoelectric body having at least three thin film layers,the layers having at least two different electromechanical couplingcoefficients. Among the layers of the multilayer piezoelectric body, acentral layer thereof has the largest electromechanical couplingcoefficient. This facilitates efficient concentration of energy of thesurface acoustic wave on a surface, so that the electromechanicalcoupling coefficient is made to be far larger than that of eachpiezoelectric body constituting each layer.

With the surface acoustic wave amplifier using the semiconductor layerof the present invention, the amplifying effect can be achieved atpractical low voltages at which a portable appliance or device is used.Moreover, a far larger amplification gain can be achieved using themultilayer piezoelectric body of the present invention.

Furthermore, with respect to the surface acoustic wave convolver, theinteraction between the surface acoustic wave convoluted and electronsis strengthened because of a high electron mobility of the semiconductorlayers, resulting in a gain larger than that of a conventionalstructure.

Furthermore, when the surface acoustic wave functional element of thepresent invention, which has a large amplification gain at practical lowvoltages, is used as a device for (i) a bandpass filter and a low noiseamplifier, (ii) a bandpass filter and a power amplifier, or (iii) abandpass filter, amplifiers, and a duplexer, in a mobile communicationapparatus, the mobile communication apparatus can be markedlydown-sized, thinned and lightened in weight. Therefore, thetransmitting/receiving circuit of a mobile communication apparatus suchas a portable telephone or a cordless telephone falls within the rangeof the present invention where the surface acoustic wave functionalelement having the high amplification gain is formed as an amplifier anda bandpass filter, or an amplifier, a bandpass filter, and a duplexer.

The present invention will be described in detail below. FIGS. 1A and 1Billustrate is shows a basic surface acoustic wave functional element ofthe present invention. FIG. 1A is a cross sectional view showing asurface acoustic wave functional element of the present invention, andFIG. 1B is a perspective view showing the surface acoustic wavefunctional element of the present invention. Reference numerals 1, 2, 3,4 and 5 designate a piezoelectric substrate, a buffer layer, an activelayer, an input electrode and an output electrode, respectively.

According to the present invention, on the piezoelectric substrate 1 arearranged the input and output electrodes 4 and 5 at a distance from eachother between which the active layer 3 is formed on the piezoelectricsubstrate 1 via the buffer layer 2.

In the present invention, the piezoelectric substrate 1 may be apiezoelectric single crystal substrate, or a piezoelectric thin filmsubstrate. For the piezoelectric single crystal substrate, anoxide-based piezoelectric substrate is preferable. For example, LiNbO₃,LiTaO₃, or Li₂ B₄ O₇ is preferably used. Moreover, a substrate cutsurface of LiNbO₃ of 64° Y cut, 41° Y cut, or 128° Y cut, or Y cut, orLiTaO₃ of 36° Y cut can be used preferably. The piezoelectric thin filmsubstrate has the structure in which a piezoelectric thin film is formedon a single crystal substrate of sapphire or Si, etc. Preferred thinfilm materials for the piezoelectric thin film include, for example,ZnO, LiNbO₃, LiTaO₃, PZT, PbTiO₃, BaTiO₃ or Li₂ B₄ O₇. Furthermore, adielectric film such as SiO, SiO₂, etc. can be inserted between a Sisubstrate and the above piezoelectric thin film. As the piezoelectricthin film substrate, a multilayer film can be formed which is fabricatedby growing the above piezoelectric thin films of different types oneabove the other on the single crystal substrate of sapphire, Si, etc.

When the piezoelectric substrate 1 of the present invention comprises amultilayer piezoelectric element of at least three layers having atleast two different electromechanical coupling coefficient and in whicha piezoelectric film located in a central portion of the multilayerpiezoelectric body has the largest electromechanical couplingcoefficients, large electromechanical coupling coefficients can beobtained.

With respect to the multilayer piezoelectric substrate, an example ofthe three-layer structure will be described in detail below referring toFIG. 2. The multilayer piezoelectric substrate 20 of the presentinvention has the structure in which there are provided on apiezoelectric substrate 21 a first piezoelectric film 22 and a secondpiezoelectric film 23. Here, the electromechanical coupling coefficientsof the piezoelectric substrate 21, the first piezoelectric film 22 andthe second piezoelectric film 23 are assumed to be k, k₁, and k₂,respectively, and the velocities of Rayleigh waves of the piezoelectricsubstrate 21, the first piezoelectric film 22 and the secondpiezoelectric film 23 are assumed to be V, V₁ and V₂, respectively. Thefilm thicknesses of the first and second piezoelectric films 22 and 23are assumed to be h₁ and h₂, respectively. Then, it is necessary that k₁is larger than k and k₂, and preferably k₁ is larger than k and k₂ by afactor of 1.2 or more, more preferably by a factor of 2 or more.Moreover, when k₁ is greater than k and k₂ and V₁ is greater than V andV₂, a far larger electromechanical coupling coefficient can be obtained.V₁ is preferably larger than V and V₂ by 100 m/s, and more preferably by250 m/s. Moreover, h₁ is normally equal to or more than 30 nm and equalto or less than 20 μm, and more preferably equal to or more than 80 nmand equal to or less than 5 μm, far more preferably equal to or morethan 100 nm and equal to or less than 2 μm. In general, h₁ /h₂ is equalto or more than 0.1 and equal to or less than 500, preferably equal toor more than 0.15 and equal to or less than 50, and more preferablyequal to or more than 0.5 and equal to or less than 21. When thewavelength of the surface acoustic wave is λ, h₁ /λ is 1 or less and h₂/λ is 1 or less, preferably h₁ /λ is 0.5 or less and h₂ /λ is 0.4 orless, more preferably h₁ /λ is 0.25 or less and h₂ /λ is 0.25 or less.

The multilayer piezoelectric substrate 20 of a large electromechanicalcoupling coefficient of the present invention is preferably used inorder to improve characteristics of a surface acoustic wave element ofnot only a surface acoustic wave amplifier and an acoustic surfaceconvolver but also a surface acoustic wave filter, a surface acousticwave resonator, etc.

As the active layer which constitutes the semiconductor layer of thepresent invention, one having a large electron mobility is preferablyused. Preferred examples of the semiconductor film which constitutes theactive layer include GaAs, InSb, InAs, and PbTe. Not only binarycompound semiconductors but also ternary and quaternary mixed crystalsderived from a combination of these binary semiconductors are preferablyused. For example, ternary mixed crystals are In_(x) Ga_(1-x) As,InAs_(y) Sb_(1-y), In_(z) Ga_(1-z) Sb and quaternary mixed crystals areIn_(x) Ga_(1-x) As_(y) Sb_(1-x) etc. In-containing semiconductor thinfilms such as those made of InAs, InSb, InAsSb, InGaSb, InGaAsSb, etc.are used preferably since they have very large electron mobilities.Moreover, the active layer may be a multilayer film formed by stackingsemiconductor films of different compositions. The electron mobility ofthe active layer is preferably 5,000 cm² /Vsec or more so as to have alarge amplification gain of the surface acoustic wave amplifier, andmore preferably, 10,000 cm² /Vsec or more so as to have a very goodamplification gain. In order to obtain this large electron mobility, theactive layer has a composition In_(x) Ga_(1-x) As where "x" can range0≦x≦1.0, preferably 0.5≦x≦1.0, and more preferably, 0.8≦x≦1.0. The largeelectron mobility can be obtained when "y" of InAs_(y) sb_(1-x) ranges0≦y≦1.0, and more preferably 0.5≦y≦1.0. "z" of In_(z) Ga_(1-z) Sbpreferably ranges 0≦z≦1.0, and more preferably, 0.8≦z≦1.0.

Moreover, when a film thickness, h, of the active layer is 5 nm or less,its crystal characteristics is deteriorated and large electron mobilitycannot be obtained. On the other hand, when h is 500 nm or more, theresistance of the active layer is lowered and at the same time theinteraction efficiency of a surface acoustic wave and carriers isdecreased. That is, in order to achieve large electron mobility and toperform the interaction of a surface acoustic wave and carriersefficiently, it is necessary that film thickness, h, of the active layerranges 5 m≦h≦500 nm, preferably 10 nm≦h≦350 nm, and more preferably 12nm≦h≦200 nm. Moreover, a sheet resistance value of the active layer ispreferably 10 Ω or more, more preferably 50 Ω or more, and mostpreferably 100 Ω or more.

It is preferable that the buffer layer formed on the piezoelectricsubstrate of the present invention be insulated or semi-insulated.However, the large resistance value is available. For example, theresistance value of the buffer layer is at least 5 to 10 or more timesas large as that of the active layer, preferably 100 times or more andmore preferably 1000 times or more are preferable examples.

As the large resistance buffer layer, for example, a binary crystal suchas GaSb, AlSb, ZnTe, or CdTe is preferably used. Ternary crystals suchas AlGaSb, AlAsSb, GaAsSb, and AlInSb are used preferably. Quaternarycrystals such as AlGaAsSb, AlInGaSb, AlInAsSb, AlInPSb, and AIGaPSb areused preferably. Moreover, when the composition of the buffer layer isdetermined, the lattice constant of the buffer layer is adjusted suchthat it has the same as or similar to the lattice constant of thecrystal constituting the active layer. Thus, large electron mobility ofthe active layer can be achieved. Here, when the difference of thelattice constant of the crystals of the active layer and that of thebuffer layer is ±7% or less, preferably ±5% or less and more preferably±2% or less, the both lattice constants are similar to each other.

Further, during the actual step of forming the buffer layer 2 on thepiezoelectric substrate 1, the lattice relaxation takes place extremelyrapidly, particularly, in a buffer layer containing Sb. Even if itslattice mismatch with the piezoelectric substrate 1 is great, thelattice disorder is relaxed merely by growing an ultra-thin film of thebuffer layer, and the buffer layer 2 begins to grow at a unique latticeconstant specific to the crystal which constitutes the buffer layer 2.Immediately before the growth of the active layer, the buffer layersurface is in extremely satisfactory conditions, thereby greatlyimproving the crystallinity of the active layer 3 formed on the bufferlayer 2. For this reason, an Sb-containing compound semiconductor isespecially suitable for use as the buffer layer 2.

The thicker the buffer layer 2, the better its crystallinity. However,it is preferred the buffer layer 2 is as thin as possible from thestandpoint of facilitation of the interaction between the surfaceacoustic wave and the carrier. More specifically, a preferred filmthickness h₃ of the buffer layer is 10 nm≦h₃ ≦1,000 nm, and preferably,20 nm≦h₃ ≦500 nm.

Further, since the buffer layer 2 can grow at low temperatures, it ispossible not only to prevent the piezoelectric substrate 1 fromdeteriorating due to leakage of oxygen, but also to prevent the activelayer 3 formed on the buffer layer 2 from deteriorating due to migrationof oxygen from the piezoelectric substrate 1. Further, the buffer layer2 of the present invention is remarkably featured in that it serves as aprotective layer for protecting the piezoelectric substrate 1 and theactive layer 3, thus eliminating the need for the provision of aprotective layer in the form of a dielectric film composed of SiO orSiO₂.

Instead, no problem arises even if a dielectric film is present betweenthe piezoelectric substrate 1 and the buffer layer 2. Examples ofmaterials used for the dielectric film are SiO, SiO₂, silicon nitride,CeO₂, CaF₂, BaF₂, SrF₂, TiO₂, Y₂ O₃, ZrO₂, MgO, and Al₂ O₃. Thedielectric film is as thin as possible. Preferably, the thickness of thedielectric film is 100 nm or less, and more preferably, 50 nm or less.

The buffer layer 2 of the present invention, as compared with adielectric film 9 such as SiO inserted between the piezoelectricsubstrate 1 and the active layer 3' of a conventional monolithic typeamplifier, is lattice-matched to the active layer and has a largedielectric constant unexpected for semiconductors and a highresistivity. Accordingly, the electric field of the surface acousticwave attenuates much less in the buffer layer 2 of the presentinvention. Therefore, the interaction between the electric field of thesurface acoustic wave and the carriers in the active layer 3 occurs moreefficiently than conventionally so that the buffer layer 2 of thepresent invention can be made thicker than the conventional dielectricfilm 9.

Further, a dielectric film or a semiconductor film may be grown on theactive layer 3 as a protective layer in order to protect the activelayer 3. As the dielectric film, the compositions indicated above may beused. As the semiconductor film, the same composition as that of thebuffer layer may be used.

Generally, any method may be employed for forming films such as thebuffer layer 2 and the active layer 3 as far as it allows growth of athin film. It is especially preferred to employ a molecular beam epitaxy(MBE) method, a metal organic molecular beam epitaxy (MOMBE) method, ametal organic chemical vapor depposition (MOCVD) method, and an atomiclayer epitaxy (ALE) method.

Further, in the present invention, the input and output electrodes 4 and5 on the piezoelectric substrate 1 are electrodes having an interdigitalstructure. For such electrodes, an apodised transducer, a withdrawalweighted transducer, a unidirectional transducer, a normal typetransducer and the like can be used. Particularly, the unidirectionaltransducer can reduce a loss due to bi-directional property of thesurface acoustic wave. Consequently, the unidirectional transducer isused most preferably. Though materials for an input electrode 4 and anoutput electrode 5 are not particularly limited, it is preferred to useAl, Au, Pt, Cu, an Al--Ti alloy, an Al--Cu alloy, an Al and Timultilayer electrode, for example.

In the case where the buffer layer 2 is formed such that the input andoutput electrodes 4 and 5 are embedded, the input and output electrodes4 and 5 are formed in the first step. Thus eliminating the need toconsider a recess and a projection of the semiconductor thin film allowsthe ultra-fine processing of the electrodes of the order of submicronsor less by the contact exposure.

However, since the input and output electrodes 4 and 5 are embedded inthe buffer layer 2, it is necessary to select electrode materials whichdeform, melt, or diffuse in the least degree during the process of theformation of the buffer layer. Preferred examples of the electrodematerials include Pt, Au, Cu, Al, Cr, Mo, Ni, Ta, Ti, W, and the like.Further, it is also preferred to use a multi-layered electrode ofTi--Pt, Ti--Al, Ti--Au, Cr--Au, Cr--Pt, etc.

There is no specific limitation to materials used as electrodes 6 forapplying the DC electric field to the semiconductor layer in the surfaceacoustic wave amplifier of the present invention. Preferred examples ofthe electrode materials include Al, Au, Ni/Au, Ti/Au, Cu/Ni/Au,AuGe/Ni/Au, and the like.

When the surface acoustic wave functional element is used for a portionof a power amplifier which is required to have the capability of highpower withstanding property, input and output electrodes 4 and 5 mustuse electrode materials which can withstand large power, i.e. power ofan order of several Watts. Examples of preferred high power resistantmaterials for such electrodes capable of withstanding large powerinclude an epitaxial Al film, an Al--Cu film, and Al--Cu/Cu/Al--Cumulti-layered film, a Ti-added-Al film, a Cu-added-Al film, aPd-added-Al film.

The surface acoustic wave amplifier of the present invention may havethe structure in which at least two separate semiconductor layers aresuccessively formed on the substrate side by side between the input andoutput electrodes 4 and 5, and in addition, there is provided thearrangement which removes carriers moving in the direction opposite tothe propagation direction of a surface acoustic wave or an arrangementin which the active layer 3 between the semiconductor layers is removed.With such a structure, the present invention can attain a highamplification gain at low voltages. For example, as shown in FIG. 6,semiconductor layers are separated by a mesa etching technique, or afterthe mesa etching process, a dielectric substance (not shown) is filledbetween the semiconductor layers so that the carriers moving in theopposite direction due to a reverse electric field can be removed.

In the surface acoustic wave convolver of the present invention, theelectrodes formed on the piezoelectric substrate are used as two inputelectrodes. Further, the signal after convolution of the surfaceacoustic wave is taken out from take-out electrodes formed on an upperportion of the semiconductor layer and on a lower portion of thepiezoelectric substrate, respectively. Materials for the take-outelectrodes are not limited specifically. Preferably, Al, Au, Pt, Cu, andthe like are used as take-out electrode materials.

In a conventional portable telephone set schematically shown in FIG. 7,to an antenna 10 is connected a duplexer 11, which is connected to areceiving amplifier 12 and a transmitting amplifier 13, each of thereceiving and transmitting amplifiers 12 and 13 connected to a band passfilter 14. In contrast, as shown in FIG. 8, when the surface acousticwave functional element capable of amplifying at a high amplificationgain according to the present invention is applied to an RF portion,only a surface acoustic wave amplifier 15 for receiving and a surfaceacoustic wave amplifier 16 for transmitting are connected to the antenna10. Therefore, according to the present invention, the number ofcomponent parts of the RF portion can be reduced as shown in FIG. 8, andeach of the component parts can be made compact, lightweight and thin.Namely, the present invention can provide compact, lightweight terminalsof portable appliances at low costs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross sectional view showing a surface acoustic wavefunctional element in accordance with an embodiment of the presentinvention;

FIG. 1B is a perspective view showing a surface acoustic wave functionalelement in accordance with an embodiment of the present invention;

FIG. 2 is a cross sectional view showing a multilayer piezoelectricsubstrate of the present invention;

FIG. 3 is a cross sectional view showing a conventional direct typeamplifier;

FIG. 4 is a cross sectional view showing a conventional separate typeamplifier;

FIG. 5 is a cross sectional view showing a conventional monolithicamplifier;

FIG. 6 is a cross sectional view showing a semiconductor layer-separatedtype surface acoustic wave amplifier in accordance with an embodiment ofthe present invention;

FIG. 7 is a schematic diagram illustrating an RF portion of a portablephone;

FIG. 8 is a schematic diagram illustrating a transmitting/receivingcircuit formed without using a duplexer and amplifiers in accordancewith the present invention;

FIG. 9 is a cross sectional view showing a surface acoustic waveamplifier in which input and output electrodes are embedded in a bufferlayer;

FIG. 10 is a cross sectional view showing a surface acoustic waveamplifier using a multilayer piezoelectric body in accordance with thepresent invention; and

FIG. 11 is a cross sectional view showing a surface acoustic waveconvolver in accordance with an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, the present invention will be described below with referring tospecific embodiments.

Embodiment 1

After SiO₂ of 10 nm thick was deposited by evaporation on a LiNbO₃single crystal substrate of 64 degree Y-cut, with a diameter of 3inches, Al₀.38 In₀.62 Sb was grown by the MBE method to 150 nm as bufferlayer. Thereafter, InSb was grown to 50 nm as an active layer.Subsequently, GaSb was grown to 2 nm as a protective layer, thus forminga semiconductor layer. This semiconductor layer had an electron mobilityof 32,000 cm² /Vs. Here, the electron mobility of the semiconductorlayer was measured by the Van der Pauw method.

Next, the semiconductor layer at a predetermined position was removed byan etching process to expose a portion of the piezoelectric substrate.Interdigital Al electrodes were formed on the surface of thepiezoelectric substrate by a lithographic process as input/outputelectrodes. The electrodes are of a normal type having a pitch of 0.75μm, with a propagation length of 300 μm. Subsequently, electrodes forapplying a DC electric field were formed on the active layer. It ispreferred that the propagation length of a surface acoustic wave is setup to a value obtained by multiplying the length of the surface acousticwave with a positive integer.

Next, the characteristics of the surface acoustic wave amplifier weremeasured. Its amplification gain was evaluated by finding a differencebetween a gain (or an insertion loss) after application of the electricfield and an insertion loss before application of the electric fieldusing a network analyzer (Yokokawa Hewlett Packard, 8510B). As a resultof the evaluation of the surface acoustic wave amplifier in accordancewith Embodiment 1 of the present invention, the amplification gain was22 dB where a DC applying voltage was 3 V, and a central frequency was1,520 MHz. This value of the amplification gain is suitable for use in alow-noise amplifier and a bandpass filter in a high frequency portion ofportable appliances.

Embodiment 2

The same sample as that of Embodiment 1 was etched to remove apredetermined portion of the semiconductor and expose a portion of thepiezoelectric substrate. Interdigital Al electrodes were formed as inputand output electrodes by lithography. The electrodes were of a normaltype electrode having a pitch of 1.4 μm, with a propagation length of280 μm. Subsequently, electrodes for applying a DC electric field wereformed on the active layer.

Next, the characteristics of the surface acoustic wave amplifier weremeasured. Its amplification gain was evaluated in the same manner as inEmbodiment 1. As a result of the evaluation of the surface acoustic waveamplifier of Embodiment 2 of the present invention, the amplificationgain was 12 dB where a DC applying voltage was 3 V, and a centralfrequency was 800 MHz. This value of the amplification gain is suitablefor use in a low-noise amplifier and a bandpass filter in a highfrequency portion of portable appliances.

Comparative Embodiment 1

As a comparison with Embodiment 2, Comparative Embodiment 1 was run.After SiO₂ of 10 nm was deposited by evaporation on a LiNbO₃ singlecrystal substrate of 64 degree Y-cut, with a diameter of 3 inches, InSbwas grown to 50 nm by a MBE method as an active layer. Thereafter, GaSbwas grown to 2 nm as a protective layer, thus forming a semiconductorlayer. The electric characteristics of the semiconductor layer weremeasured. However, in this Comparative Embodiment, InSb as the activelayer was directly formed on the piezoelectric substrate via the SiO₂film. Consequently, the crystal property of the active layer wasinsufficient, and its electron mobility was only 1,700 cm² /Vs.

Thereafter, the semiconductor layer at a predetermined position wasetched to expose the piezoelectric substrate. Interdigital Al electrodeswere formed on the surface of the piezoelectric substrate by alithographic process as input and output electrodes, respectively. Theelectrodes were of a normal type having a pitch of 1.4 μm, with apropagation length of 280 μm. Subsequently, electrodes for applying a DCelectric field were formed on the active layer. Thereafter, the surfaceacoustic wave amplifying characteristics were measured similarly as inEmbodiment 2, but no amplifying effect was observed.

Embodiment 3

After SiO₂ of 10 nm was deposited by evaporation on a LiNbO₃ singlecrystal substrate of 64 degree Y-cut, with a diameter of 3 inches, Al₀.5Ga₀.5 Sb was grown to 200 nm by a MBE method as a buffer layer.Thereafter, InAs₀.5 Sb₀.5 was grown to 60 nm as an active layer.Subsequently, GaSb was grown to 2 nm as a protective layer, thus forminga semiconductor layer. This semiconductor layer had an electron mobilityof 30,000 cm² /Vs.

Next, the semiconductor layer at predetermined portion was etched toexpose the piezoelectric substrate. Interdigital Al electrodes wereformed on the surface of the piezoelectric substrate by a lithographicprocess as input and output electrodes, respectively. The electrodeswere of a normal type having a pitch of 0.6 μm, with a propagationlength of 240 μm. Subsequently, electrodes for applying a DC electricfield were formed on the active layer.

Next, the characteristics of a surface acoustic wave amplifier weremeasured. Its amplification gain was evaluated by the same method as inEmbodiment 1. As a result of the evaluation of the surface acoustic waveamplifier of Embodiment 3 of the present invention, the amplificationgain was 26 dB where a DC applying voltage was 3 V, and a centralfrequency was 1,900 MHz. This value of the amplification gain issuitable for use in a low-noise amplifier and a bandpass filter in ahigh frequency portion of portable appliances.

Embodiment 4

After SiO₂ of 10 nm was deposited by evaporation on a LiNbO₃ singlecrystal substrate of 64 degree Y-cut, with a diameter of 3 inches, Al₀.5G₀.5 Sb was grown to 150 nm by a MBE method as buffer layer. Thereafter,InAs₀.5 Sb₀.5 was grown to 50 nm as an active layer. Subsequently, GaSbwas grown to 2 nm as a protective layer, thus, forming a semiconductorlayer. This semiconductor layer had an electron mobility of 20,900 cm²/Vs.

Next, the semiconductor layer at a predetermined position was etched toexpose the piezoelectric substrate. Interdigital Al electrodes wereformed on the surface of the piezoelectric substrate by a lithographicprocess as input and output electrodes. respectively. The electrodes areof a normal type having a pitch of 0.75 μm, with a propagation length of300 μm. Subsequently, electrodes for applying a DC electric field wereformed on the active layer.

Next, the characteristics of a surface acoustic wave amplifier weremeasured. Its amplification gain was evaluated by the same method as inEmbodiment 1. As a result of the evaluation of the surface acoustic waveamplifier of Embodiment 4 of the present invention, the amplificationgain was 13 dB where a DC applying voltage was 3 V, and a centralfrequency was 1,530 MHz. This value of the amplification gain issuitable for use in a low-noise amplifier and a bandpass filter in ahigh frequency portion of portable appliances

Embodiment 5

After SiO₂ of 10 nm was deposited by evaporation on a LiNbO₃ singlecrystal substrate of 64 degree Y-cut, with a diameter of 3 inches, Al₀.5Ga₀.5 Sb was grown to 100 nm by a MBE method as a buffer layer.Thereafter, InAs₀.5 Sb₀.5 was grown to 200 nm as an active layer.Subsequently, GaSb was grown to 2 nm as a protective layer, thus forminga semiconductor layer. This semiconductor layer had an electron mobilityof 32,000 cm² /Vs.

Next, the semiconductor layer at a predetermined position was etched toexpose a piezoelectric substrate. Interdigital Al electrodes were formedon the surface of the piezoelectric substrate by a lithographic processas input and output electrodes, respectively. The electrodes were of anormal type having a pitch of 0.75 μm, with a propagation length of 300μm. Subsequently, electrodes for applying a DC electric field wereformed on the active layer.

Next, the characteristics of the surface acoustic wave amplifier weremeasured. Its amplification gain was evaluated by the same method as inEmbodiment 1. As a result of the evaluation of the surface acoustic waveamplifier of Embodiment 5 of the present invention, the amplificationgain was 6 dB where a DC applying voltage was 6 V, and a centralfrequency was 1,505 MHz. Consequently, the amplifying effect wasobtained at a voltage as low as 6 V.

Comparative Embodiment 2

As a comparison with Embodiment 4, Comparative Embodiment 2 was run.After SiO₂ of 10 nm was deposited by evaporation on a LiNbO₃ singlecrystal substrate of 64 degree Y-cut, with a diameter of 3 inches,InAs₀.5 Sb₀.5 was grown to 50 nm by a MBE method as an active layer.Thereafter, GaSb was grown to 2 nm as protective layer, thus forming asemiconductor layer. The electric characteristics of this semiconductorlayer were measured. However, in this Comparative Embodiment, InAs₀.5Sb₀.5 as the active layer was directly formed on the piezoelectricsubstrate via the SiO₂ film. Consequently, the crystallinity of theactive layer was inadequate, and its electron mobility was only 1,200cm² /Vs.

Thereafter, the semiconductor layer at a predetermined position wasetched to expose the piezoelectric substrate. Interdigital Al electrodeswere formed on the surface of the piezoelectric substrate by alithographic process as input and output electrodes, respectively. Theelectrodes were of a normal type having a pitch of 0.75 μm, with apropagation length of 300 μm. Subsequently, electrodes for applying a DCelectric field were formed on the active layer. Thereafter, the surfaceacoustic wave amplifying characteristics were measured, but noamplifying effect was observed.

Embodiment 6

After SiO₂ of 10 nm was deposited by evaporation on a LiNbO₃ singlecrystal substrate of 64 degree Y-cut, with a diameter of 3 inches, Al₀.5Ga₀.5 Sb was grown to 150 nm by a MBE method as a buffer layer.Thereafter, InAs was grown to 350 nm as an active layer. Subsequently,GaSb was grown to 2 nm as a protective layer, thus forming asemiconductor layer. This semiconductor layer had an electron mobilityof 22,000 cm² /Vs.

Next, the semiconductor layer at a predetermined position was etched toexpose a piezoelectric substrate. Interdigital Al electrodes were formedon the surface of the piezoelectric substrate by a lithographic processas input and output electrodes, respectively. The electrodes were of anormal type having a pitch of 0.6 μm, with a propagation length of 240μm. Subsequently, electrodes for applying a DC electric field wereformed on the active layer.

Next, the characteristics of the surface acoustic wave amplifier weremeasured. Its amplification gain was evaluated by the same method as inEmbodiment 1. As a result of the evaluation of the surface acoustic waveamplifier of Embodiment 6, the amplification gain was 2 dB where a DCapplying voltage was 6 V, and a central frequency was 1,500 MHz.Consequently, the amplifying effect was obtained at a voltage as low as6 V.

Embodiment 7

After SiO₂ of 10 nm was deposited by evaporation on a LiNbO₃ singlecrystal substrate of 64 degree Y-cut, with a diameter of 3 inches, Al₀.5Ga₀.5 As₀.12 Sb₀.88 was grown to 150 nm by a MBE method as a bufferlayer. Thereafter, InAs was grown to 50 nm as an active layer,Subsequently, GaSb was grown to 2 nm as a protective layer, thus forminga semiconductor layer. This semiconductor layer has an electron mobilityof 13,000 cm² /Vs.

Next, the semiconductor layer at a predetermined position was etched toexpose a piezoelectric substrate. Interdigital Al electrodes were formedon the surface of the piezoelectric substrate by a lithographic processas input and output electrode, respectively. The electrodes were of anormal type having a pitch of 1.4 μm, with a propagation length of 560μm. Subsequently, electrodes for applying a DC electric field wereformed on the active layer.

Next, the characteristics of the surface acoustic wave amplifier weremeasured. Its amplification gain was evaluated by the same method as inEmbodiment 1. As a result of the evaluation of the surface acoustic waveamplifier of Embodiment 7, the amplification gain was 6 dB where a DCapplying voltage was 5 V, and a central frequency was 810 MHz.Consequently, the amplifying effect was obtained at a voltage as low as5 V.

Embodiment 8

After SiO₂ of 10 nm was deposited by evaporation on a LiNbO₃ singlecrystal substrate of 64 degree Y-cut, with a diameter of 3 inches, Al₀.5Ga₀.5 As₀.12 Sb₀.88 was grown to 150 nm by a MBE method as a bufferlayer. Thereafter, InAs was grown to 20 nm as an active layer.Subsequently, GaSb was grown to 2 nm as a protective layer, thus forminga semiconductor layer. This semiconductor layer had an electron mobilityof 8000 cm² /Vs.

Next, the semiconductor layer at a predetermined position was etched toexpose a piezoelectric substrate. Interdigital Al electrodes were formedon the surface of the piezoelectric substrate by a lithographic processas input and output electrodes, respectively. The electrodes are of anormal type having a pitch of 1.4 μm, with a propagation length of 560μm. Subsequently, electrodes for applying a DC electric field wereformed on the active layer.

Next, the characteristics of the surface acoustic wave amplifier weremeasured. Its amplification gain was evaluated by the same method as inEmbodiment 1. As a result of the evaluation of the surface acoustic waveamplifier of Embodiment 8, the amplification gain was 3 dB where a DCapplying voltage was 6 V, and a central frequency was 835 MHz.Consequently, the amplifying effect was obtained at a voltage as low as6 V.

Embodiment 9

After SiO₂ of 10 nm was deposited by evaporation on a LiNbO₃ singlecrystal substrate of 64 degree Y-cut, with a diameter of 3 inches, Al₀.5Ga₀.5 As₀.12 Sb₀.88 was grown to 150 nm by a MBE method as a bufferlayer. Thereafter, InAs was grown to 10 nm as an active layer.Subsequently, GaSb was grown to 2 nm as a protective layer, thus forminga semiconductor layer. This semiconductor layer had an electron mobilityof 5,000 cm² /Vs.

Next, the semiconductor layer at a predetermined position was etched toexpose a piezoelectric substrate. Interdigital Al electrodes were formedon the surface of the piezoelectric substrate by a lithographic processas input and output electrodes, respectively. The electrodes were of anormal type having a pitch of 0.75 μm, with a propagation length of 300μm. Subsequently, electrodes for applying a DC electric field wereformed on the active layer.

Next, the characteristics of the surface acoustic wave amplifier weremeasured. An amplification gain was evaluated by the same method as inEmbodiment 1. As a result of the evaluation of the surface acoustic waveamplifier of Embodiment 9, the amplification gain was 4 dB where a DCapplying voltage was 6 B, and a central frequency was 1,560 MHz.Consequently, the amplifying effect was obtained at a voltage as low as6 V.

Comparative Embodiment 3

As a comparison with Embodiment 7, Comparative Embodiment 3 was run.After SiO₂ of 10 nm was deposited by evaporation on a LiNbO₃ singlecrystal substrate of 64 degree Y-cut, with a diameter of 3 inches, InAswas grown to 50 nm by a MBE method as an active layer. Thereafter, GaSbwas grown to 2 nm as a protective layer, thus forming a semiconductorlayer. The electric characteristics of this semiconductor layer weremeasured. However, in this Comparative Embodiment, InAs as the activelayer was directly formed on the piezoelectric substrate via the SiO₂film. Consequently the crystallinity of the active layer was inadequate,and its electron mobility was only 900 cm² /Vs. Thereafter, asemiconductor layer at a predetermined position was etched to expose thepiezoelectric substrate. Interdigital Al electrodes were formed on thesurface of the piezoelectric substrate by a lithographic process asinput and output electrodes, respectively. The electrodes were of anormal type having a pitch of 1.4 μm, with a propagation length of 560μm. Subsequently, electrodes for applying a DC electric field wereformed on the active layer. Thereafter, surface acoustic wave amplifyingcharacteristics were measured, but no amplifying effect was observed.

Embodiment 10

After Al₀.5 Ga₀.5 As₀.1 Sb₀.9 as a buffer layer was grown to 50 nm on aLiNbO₃ single crystal substrate of 128 degree Y-cut, with a diameter of3 inches by a MBE method, InSb was grown to 400 nm as an active layer,thus forming a semiconductor layer. This semiconductor layer had anelectron mobility of 7,000 cm² /Vs and a carrier concentration of 1×10¹⁶cm⁻³.

Next, the surface acoustic wave amplifier was manufactured by the samemethod as in Embodiment 1. The electrodes were of a normal type having apitch of 0.75 μm, with a propagation length of 300 μm.

As a result of measurement of the characteristics of the surfaceacoustic wave amplifier, an amplification gain was 3 dB where a DCapplying voltage was 5 V, and a central frequency was 1,500 MHz.Consequently, the amplifying effect was obtained at a voltage as low as5 V. Furthermore, the deterioration of the LiNbO₃ substrate ordeterioration of the InSb active layer due to oxygen diffusion from theLiNbO₃ substrate was not observed even without any dielectric film ofSiO. As a result, it was confirmed that the Al₀.5 Ga₀.5 As₀.1 Sb₀.9buffer layer functioned as a protective film for the semiconductoractive layer and a piezoelectric substrate.

Comparative Embodiment 4

As a comparison with Embodiment 10, Comparative Embodiment 4 was run.InSb as an active layer was grown to 50 nm by a MBE method directly on aLiNbO₃ single crystal substrate of 64 degree Y-cut, with a diameter of 3inches, thus forming a semiconductor layer. The electric characteristicsof this semiconductor layer were measured. In this ComparativeEmbodiment, since InSb was grown directly on the LiNbO₃ substratewithout forming a protective layer such as a dielectric film, thequality of the InSb film as the active layer was deteriorated due tooxygen leakage from LiNbO₃. Consequently, an electron mobility could notbe measured.

Embodiment 11

A surface acoustic wave amplifier having a cross sectional structure asshown in FIG. 9 was fabricated.

Interdigital Ti--Pt electrodes as input and output electrodes 4 and 5were formed at a predetermined position on a LiNbO₃ single crystalsubstrate 1 of 128 degree Y-cut, with a diameter of 3 inches, by alithographic process of normal contact exposure. The electrodes 4 and 5were of a normal type having a pitch of 1.4 μm, with a propagationlength of 364 μm. Subsequently, Al₀.38 In₀.62 Sb was grown to 150 nm bya MBE method as a buffer layer 2 in such a manner that the input andoutput interdigital electrodes 4 and 5 were embedded therewith on thesubstrate 1. Thereafter, InSb was grown to 50 nm as an active layer 3,thus forming a semiconductor layer. This semiconductor layer had anelectron mobility of 34,000 cm² /Vs.

Next, after the active layer 3 at a predetermined position was removedby an ion milling method, electrodes 6 for applying a DC electric fieldwere formed on the active layer 3 (FIG. 9 shows a cross sectional viewfor the structure thereof). Thereafter, a silicon nitride film forpassivation was deposited by evaporation, and then, windows were formed.The characteristics of the surface acoustic wave amplifier weremeasured. As a result, the amplification gain was as large as 17 dBwhere a DC applying voltage was 6 V, and a central frequency was 809MHz.

Embodiment 12

A surface acoustic wave amplifier having a cross sectional structure asshown in FIG. 10 was fabricated.

LiNbO₃ layer 17 having a thickness of 2.0 μm was grown on a LiTaO₃single crystal substrate 1 of Y-cut by a laser ablation method, andfurther, a LiTaO₃ layer 18 having a thickness of 0.1 μm was grown on theLiNbO₃ layer 17, thus forming a multilayer piezoelectric substrate of athree-layer structure. As a result of an analysis of the grown thin filmby Auger electron spectroscopy, it was confirmed that a LiNbO₃ film 17and a LiTaO₃ film 18 without any disorder in stoichimetry was formed.Moreover, it was found by X-ray diffraction that a (110) LiNbO₃ layer 17and a (110) LiTaO₃ layer 18 were heteroepitaxially grown in a twin ordomain free state.

In order to measure the electromechanical coupling coefficient of themultilayer piezoelectric substrate, an Al comb-like electrode was formedby a common lithographic process in such a manner that the wavelength ofa surface acoustic wave became 8 μm. The electromechanical couplingcoefficient measured by a network analyzer was as large as 20.0%.

Additionally, the surface acoustic wave amplifier as depicted in FIG. 10was manufactured by the use of the multilayer piezoelectric substrate inthe same method as in Embodiment 10. As a result of measurement of thecharacteristics of the surface acoustic wave amplifier, theamplification gain was 12 dB where a DC applying voltage was 5 V and acentral frequency was 1,500 MHz. It was confirmed that the multilayerpiezoelectric substrate having a remarkably large electromechanicalcoupling coefficient in this Embodiment exhibited an excellentamplifying effect in the surface acoustic wave amplifier about fourtimes more than was obtained in Embodiment 10.

Comparative Embodiment 5

As a comparison with Embodiment 12, Comparative Embodiment 5 was run.The electromechanical coupling coefficient of the multilayerpiezoelectric substrate in Embodiment 12 was compared with theelectromechanical coupling coefficients of materials constituting eachlayer and a two-layer piezoelectric substrate. The results obtained ofelectromechanical coupling coefficients of materials constituting eachlayer in the same method as Embodiment 12 were 4.7% in the singlecrystal (110) LiNbO₃ and 0.68% in the single crystal (110) LiTaO₃.

An electromechanical coupling coefficient was measured also in the caseof constituting materials formed in a two-layer structure. Namely, aLiNbO₃ film was grown on a Y-cut LiTaO₃ substrate by the same laserablation method as in Embodiment 12. The result of measurement of anelectromechanical coupling coefficient was 3.0% in the two-layerstructure of LiNbO₃ /LiTaO₃, which was lower than the single crystal(110) LiNbO₃.

Consequently, it has been found that the electromechanical couplingcoefficient of the multilayer piezoelectric substrate according to thepresent invention can be greatly increased in the case of thethree-layer structure. The electromechanical coupling coefficient wasincreased about four times that obtained in Embodiment 12, therebyincreasing the amplification gain of the surface acoustic waveamplifier.

Embodiment 13

By the same method as in Embodiment 10, a semiconductor layer was grownon a LiNbO₃ substrate of 128 degree Y-cut. Subsequently, thesemiconductor layer at a predetermined position was etched to expose apiezoelectric substrate. Interdigital Al--Cu/Cu/Al--Cu multilayerelectrodes as input and output high-withstand power electrodes wereformed on the piezoelectric substrate by a lithographic process. Theelectrodes were of a normal type having a pitch of 0.75 μm, with apropagation length of 300 μm. Subsequently, electrodes for applying a DCelectric field were formed on the active layer.

The characteristics of the surface acoustic wave amplifier wereevaluated by sending an RF signal from a signal generator (AnritsuMG3670A) so as to measure an amplification gain and a transmission powerby means of a power meter and a power sensor (Yokokawa Hewlett Packard,437B and 8481H). The amplification gain was 22 dB and the transmissionpower was 2.2 W where a DC applying voltage was 3 V and a centralfrequency was 1,520 MHz. Consequently, the surface acoustic waveamplifier in this Embodiment can be used as an excellent power amplifierin a high frequency portion of mobile communication appliances or thelike, and further, it can remarkably contribute to down-sizing of suchappliances.

Embodiment 14

A semiconductor layer was grown on a LiNbO₃ substrate of 128 degree Ycut as well as the Embodiment 10. Next, the semiconductor layer wasprocessed to be arranged between input and output electrodes beingformed later, and also was etched so as to separate the semiconductorlayer into three portions as shown in FIG. 6. Interdigital Al electrodeswere formed on the exposed surface of the piezoelectric substrate asinput and output electrodes. Electrode pitches and the propagationwavelengths are the same values as those in Embodiment 10. Furthermore,electrodes for applying a direct current electric field was formed oneach of the separated active layer.

The characteristics of the surface acoustic wave amplifier of thepresent invention was evaluated by applying direct current electricfield in parallel to each of the active layers. As a result, theamplification gain was 8 dB where a direct current applied voltage was 5V, and a center frequency was 1,500 MHz. It was confirmed that theenhancement of the amplification gain substantially 3 times as large asthat of Embodiment 10 was achieved.

Embodiment 15

A surface acoustic wave convolver having a cross sectional structure asshown in FIG. 11 was fabricated.

A semiconductor layer was grown on a LiNbO₃ substrate 1 of 128 degree Ycut as in Embodiment 10. Next, a semiconductor layer serving a bufferlayer 2 was etched at a predetermined position to expose thepiezoelectric substrate 1. Input electrodes 4 were formed on the surfaceof the piezoelectric substrate 1 by a lithography process. Furthermore,take-out electrodes 19 were formed on the top of the semiconductor andat the bottom of the piezoelectric substrate, as shown in FIG. 11, and asurface acoustic wave convolver was produced.

In addition, using the surface acoustic wave convolver of the presentinvention, when the amplifying characteristics of frequency of 1,000 MHzwas measured by a frequency analyzer, a convolution output as anonlinear signal could be obtained from the take-out electrode 19.

Embodiment 16

Using the surface acoustic wave amplifier produced in Embodiment 1 ofthe present invention, a mixer, and a quadrature modulator, thereceiving circuit of the RF portion of a portable phone was produced. Nospecific circuit for matching the impedances was provided between thesurface acoustic wave amplifier and the mixer. Normally, for the portionconstructed with a low noise amplifier and a high-frequency band passfilter, only the surface acoustic wave amplifier was used. The RF signalwhich was π/4QPSK modulated was provided from a signal generator to thereceiving circuit of the RF portion of the portable phone produced asabove described, so that demodulation errors of I and Q output signalsafter receiving the RF signal were measured by using a vector signalanalyzer (Yokokawa Hewlett Packard 89441A). Consequently, when thestrength of input signal was between -10 and -102 dBm, the size of themaximum error vector was 16% rms. Moreover, as a result of comparison ofinput data with demodulated data, there was no error in the datademodulated. Furthermore, the noise figure and amplification gain of thesurface acoustic wave amplifier were measured by using a noise figuringmeter (Yokokawa Hewlett Packard 8970B) and a noise source (YokokawaHewlett Packard 346B). As a result, at 810 MHz, the noise figure was 2.5dB and the amplification gain was 14 dB; at 826 MHz, the noise figurewas 3 dB and the amplification gain was 12 dB; and at 815 MHz, the noisefigure was 1.8 dB and the amplification gain was 16 dB. In addition,attenuation characteristics outside of the pass band was measured byusing a network analyzer. At 940 MHz, and insertion loss was 35 dB, andat 956 MHz, and insertion loss of 40 dB was measured. In conclusion, itwas confirmed that a receiving circuit using the surface acoustic waveamplifier instead of the low noise amplifier and the band pass filtercould be achieved. Furthermore, if the surface acoustic wave amplifierof this Embodiment of the present invention was used, a highfrequency-low noise amplifier could be made monolithically and resultingin the reduction of the number of the parts of the receiving circuit.

Embodiment 17

The receiving circuit of the RF portion of a portable phone was producedby using an orthogonal modulator, a mixer, a band pass filter and asurface acoustic wave amplifier. Normally, the surface acoustic waveamplifier of the present invention is used as the part to be constructedwith a power amplifier. The RF signal which was π/4QPSK modulated isprovided from a signal generator to the surface acoustic wave amplifier,so that demodulation errors of the I and Q output signals were measuredby using a vector signal analyzer. As a result, the size of an errorvector at a center frequency of 948 MHz was 5.5% rms. In this case, atransmission electric power was 2.2 W. It was confirmed that when thespectrum of the output signal was measured by a spectrum analyzer, itmet Nippon Digital system automobile telephone system standardnormalization (RCR STD-27). In conclusion, it was confirmed that thetransmitting circuit could be produced by using the surface acousticwave amplifier instead of a conventional power amplifier to enabledown-sizing of the power amplifier portion.

Embodiment 18

The transmitting circuit of the RF portion of a portable phone similarto that obtained in Embodiment 17 was produced without using a band passfilter. As a result, the size of an error vector at a center frequencyof 948 MHz was 4.0% rms. Next, it was confirmed that the transmittingelectric power at this time was 3.2 W, and a transmitting spectrum metRCR STD-27. In conclusion, the transmitting circuit can be produced byusing the surface acoustic wave amplifier instead of a conventionalpower amplification module and a band pass filter, so that poweramplification and the band pass filter can be made monolithic.

Embodiment 19

A transmitting/receiving circuit was produced by using a surfaceacoustic wave amplifier with a pass band of 810 to 826 MHz instead ofthe low noise amplifier of a receiving circuit and the band pass filterand also using a surface acoustic wave amplifier with a pass band of 940to 956 MHz instead of the power amplifier of a transmitting circuit andthe band pass filter. The same surface acoustic wave amplifier of areceiving portion as in Embodiment 16 was used and the same surfaceacoustic wave amplifier of a transmitting portion as in Embodiment 17was used. An antenna terminal was connected to a transmitting circuitand a receiving circuit, with a micro-strip line in which thecharacteristic impedance was adjusted to be 50 ohm without using aduplexer. The receiving characteristics and the transmittingcharacteristics of the transmitting/receiving circuit produced asdescribed above were measured similarly to Embodiment 16 and Embodiment17. As a result of measurement of the receiving characteristics, thesize of the maximum error vector was 18% rms when the strength of aninput signal was between -10 and -102 dBm. Moreover, there was no errorin the data demodulated at this time. As a result of measurement of thetransmitting characteristics, the size of the error vector at a centerfrequency of 948 MHz was 5.4% rms. Moreover, it was confirmed that thetransmitting electric power at this time was 3.0 W, and a transmittingspectrum met RCR STD-27. In conclusion, it was confirmed that in thetransmit-receive circuit of the RF portion of the portable phone,circuits using the surface acoustic wave amplifier can be used insteadof the low noise amplifier and the band pass filter, instead of thepower amplification module and the band pass filter, and instead of theduplexer. Therefore, when the transmitting/receiving circuit of thisEmbodiment of the present invention was used, the number of the parts ofthe RF portion of a conventional portable communication apparatus can besufficiently reduced, so that it can contribute to drastic down-sizingand weight reduction, as well as price reduction of portable apparatusterminals.

Comparative Embodiment 6

The typical size of a power amplification module constructed with aconventional GaAsFET, a capacitor, etc. is 25 mm×12 mm×3.7 mm. Incontrast, the surface acoustic wave amplifier of Embodiment 17 is 5 mm×5mm×2 mm, so that the present invention can achieve drastic down-sizingof the conventional power amplifier.

The used of the surface acoustic wave functional element of the presentinvention permits to greatly improve the gain of a surface acoustic waveamplifier or the efficiency of a surface acoustic wave convolver. Thesurface acoustic wave amplifier of the present invention can achieve alarge amplification gain at low voltages which is practicallyadvantageous so that it is applicable to high frequency portions ofmobile communication appliances. Furthermore, the present inventionpermits to use a single component in place of an amplifier, a bandpassfilter, or a duplexer which have been used as discrete elements and areof a larger size. Thus, the present invention can contribute to theproduction of compact, lightweight, thinned mobile communicationappliances at low manufacturing costs.

What is claimed:
 1. A surface acoustic wave functional element,comprising:a piezoelectric substrate; an input electrode and an outputelectrode formed on said piezoelectric substrate; and semiconductorlayers provided between said input electrode and said output electrode,wherein said semiconductor layers include an active layer and a bufferlayer, said buffer layer comprises a compound which contains at leastantimony and said buffer layer has a lattice constant matched to that ofsaid active layer.
 2. The surface acoustic wave functional element asclaimed in claim 1, wherein said active layer comprises a compound whichcontains at least indium.
 3. The surface acoustic wave functionalelement as claimed in claim 1, wherein said active layer has a filmthickness that is from 5 nm to 500 nm inclusive.
 4. The surface acousticwave functional element as claimed in claim 1, further comprising aunidirectional transducer for input-output power conversion.
 5. Thesurface acoustic wave functional element as claimed in claim 1, whereinsaid surface acoustic wave functional element is formed as part of anamplifier and further comprises electrodes for applying a direct currentelectric field to said semiconductor layer.
 6. The surface acoustic wavefunctional element as claimed in claim 5, wherein said semiconductorlayer is divided into two or more blocks between said input and outputelectrodes and wherein said functional element has an arrangement inwhich carriers moving in a reverse direction are removed between saidinput electrode and said output electrode or an arrangement in which noactive layer is present between said two or more blocks.
 7. The surfaceacoustic wave functional element as claimed in claim 5, wherein saidinput and output electrodes are embedded in said buffer layer and saidactive layer is formed on said buffer layer.
 8. The surface acousticwave functional element as claimed in claim 5, wherein said input andoutput electrodes have a high withstand power structure.
 9. Atransmitting/receiving circuit, comprising a surface acoustic wavefunctional element described in claim 5, formed as an amplifier and abandpass filter, or as an amplifier, a bandpass filter and a duplexer.10. The transmitting/receiving circuit as claimed in claim 9, whereinsaid transmitting/receiving circuit transmits or receives a signal to orfrom a mobile communication apparatus.
 11. A surface acoustic wavefunctional element comprising:a piezoelectric substrate; and input andoutput electrodes formed on said piezoelectric substrate, wherein saidpiezoelectric substrate comprises a multilayer piezoelectric bodycomprising at least three layers having at least two differentelectromechanical coupling coefficients, and wherein a central layer ofsaid at least three layers of said multilayer piezoelectric body has thelargest electromechanical coupling coefficients.
 12. The surfaceacoustic wave functional element as claimed in claim 11, wherein saidmultilayer piezoelectric body consists of three layers, said centrallayer of said piezoelectric body consists of LiNbO₃ and the rest layersof said piezoelectric body consists of LiTaO₃.
 13. The surface acousticwave functional element as claimed in claim 11, wherein a unidirectionaltransducer is used for input-output transducers.
 14. The surfaceacoustic wave functional element as claimed in claim 11, whereinsemiconductor layers exist between said input electrode and said outputelectrode.
 15. The surface acoustic wave functional element as claimedin claim 14, wherein said semiconductor layers consist of an activelayer and a buffer layer, said buffer layer having a lattice constantmatch ed to that of said active layer.
 16. The surface acoustic wavefunctional element as claimed in claim 14, further comprising aunidirectional transducer for input-output power conversion.
 17. Thesurface acoustic wave functional element as claimed in claim 14, whereinsaid surface acoustic wave functional element is formed as part of anamplifier and further comprises electrodes for applying a direct currentelectric field to said semiconductor layer.